Here is a simple circuit which can be used as a Fire Alarm. 3 Volt is enough to operate this project. There is not much to the circuit. The IC UM66 is connected to its supply and its output is fed to a transistor for amplification.
UM66 is a complete miniature tone generator with a ROM of 64 notes, oscillator and a preamplifier. For amplification we have used a NPN transistor which is BC548. Here BC548 makes a common emitter circuit. For limiting the base current we have used a resistance of 220 Ohms so that transistor will not get damaged.
For heat sensor we have used tube light starter in place of manual switch. In a starter there is a metal plate and a pin with small gap. When starter gets heated then metal plate of starter expands and get in contact with the pin and circuit is completed and we get audio from speaker. For fast sensing we can use starter without its glass body by carefully breaking glass cover.